1. Field of the Invention
This invention relates to an electric motor drive device for a camera and more particularly to an electric motor drive device of the type using a coupling means such as an electromagnetic clutch for coupling a motor and a winding mechanism.
2. Description of the Prior Art
Some of the conventional electric motor drive devices of the type using an electromagnetic clutch for controlling coupling between the motor and a winding mechanism reduce power consumption required for starting the motor by first switching off the electromagnetic clutch to start the motor under a no load condition and, then, after the lapse of a prescribed length of time, by switching on the electromagnetic clutch to accomplish winding. Others avoid a delayed start of the motor by starting it in association with the operation of a release switch of the camera, instead of starting it after completion of a photographing operation, and by thus allowing the motor to be rotating under a no load condition with the electromagnetic clutch switched off during photographing. Then, the electromagnetic clutch is switched on, upon completion of photographing, to accomplish winding by coupling the normally rotating motor with the winding mechanism. However, the ratio of current consumption between motor rotation under a no load condition and motor rotation during winding is about 1:10. Accordingly, where batteries are used as the power source, a voltage drop due to internal resistance tends to impress a higher voltage on the motor for rotation under no load than for rotation during winding. This tendency becomes more conspicuous as the number of batteries in use increases. Furthermore, the rating of the motor is generally set either at the voltage required for winding or lower than that in consideration of the working range of battery capacity, the efficiency of the motor, etc. With the rating of the motor set in this manner, a high voltage which exceeds the rating, is impressed on the motor. In a high speed type motor drive device, many batteries, say 12 to 20 batteries are used. In such a case, an excessively high voltage is impressed on the motor when it rotates under no load. The excessive voltage then results in vibration, beat, etc. This not only gives a disagreeable feeling, but also gives blurred pictures due to camera vibration.
Under a no load condition, the number of revolutions of the motor is larger than the number of revolutions during winding even if the voltage is the same. However, with the excessive voltage impressed, the number of revolutions further increases under a no load condition to eventually shorten the mechanical life of the motor.
It is an object of the invention to provide an electric motor drive device for a camera which solves the above problem of the prior art by switching over the motor drive circuit between operation under a no load condition and operation under a loaded condition in such a way as to lower the voltage impressed on the motor under a no load condition.
It is another object of the invention to provide an electric motor drive device for a camera in which a power supply control circuit for the motor thereof is switched over between operation under a no load condition and operation under a loaded condition in such a way to prevent the voltage from being impressed on the motor from increasing due to an increase in the terminal voltage of the power source, so that any excess voltage can be prevented from being impressed on the motor during operation under a no load condition.
These and further objects, features and advantages of the present invention will become apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.